1. Field of the Invention
The present invention relates to a power system stabilizer provided at an individual power system to be separated due to an accident cutting off a link line which interconnects a number of power systems, which include power stations such as power plants, transformer substations and loads, to form a large-scale power system (main system) and, more particularly, to a power stabilizer for enabling high accuracy control of a demand-supply balance in the separated power system by assuming a load-drop amount due to a voltage drop caused by the accident when the individual power system is separated from the main system.
2. Description of the Prior Art
Conventionally, for this kind of power system stabilizer, there has been proposed a power system stabilizer as described on pages 5 and 6 of a paper titled "The Protective Relaying System for Preventing Power Failure Extension in Bulk Power System", published in the International Conference on Large High Voltage Electric Systems held on Sept. 1 to 9, 1982.
The above-mentioned power system stabilizer is constructed as shown in FIG. 1. In the drawing, a whole system comprises a transformer substation 1 belonging to a main system side; a central transformer substation 2 in a separated system to be controlled; a power plant 3 similarly belonging to the same separated system. These substations and plant are interconnected via power lines 4 and 5. A reference numeral 30a shows a group of loads placed out of control and a reference numeral 30b denotes a group of loads to be controlled. A system stabilizer 6 is installed at the central transformer substation 2 and comprises an input conversion circuit 61a for measuring load power, an input conversion circuit 61b for measuring a tidal current of a link line, an input conversion circuit 61c for measuring a (power) generator output, a route-disconnection detector 62, an arithmetic processing unit 63 employing a microprocessor, a stopper 64, a trip-signal output circuit 65, an input conversion circuit 67 for measuring load voltages, etc.
Next, the operation of the whole system will be described. Current and voltage data detected by sensors 23, 24 and 32, each of which is made up of a current transformer C.T and a instrument transformer P.T, are given to the system stabilizer 6 through control cables 25 and 27 and a communication route 33. On the basis of these data, the input conversion circuits 61a, 61b and 61c, each of which comprises a filtering circuit for eliminating a harmonic component and a transient oscillation component, etc., an effective power converter for deriving effective power, an analog-to-digital (A/D) converter for converting an analogue amount into a digital one, etc., calculate the effective power components of loads to be broken and the output (effective force) of the generator and send them, after being converted into digital amounts, to the processing unit 63. Further, a load voltage V.sub.L detected by a sensor 28, which is composed of an instrument transformer PT, is converted into a digital amount at the input conversion circuit 67 comprising a filtering circuit, an A/D converter, etc. to be output to the unit 63. Opening and closing information of (circuit) breakers 11 and 21 sent through a control cable 26 and a communication route 12 is produced to the unit 63 after being converted into digital information at the route -disconnection detector 62. In the case where a route-disconnection accident has taken place in the link line with the main system and it has been detected that an object system is brought into a single operation state using those data, the arithmetic processing unit 63 performs stabilizing control according to a flow chart shown in FIG. 2.
The flow chart shown in FIG. 2 comprises a starting block 70 for starting the stabilizing control after the occurrence of a route disconnection between the main system and the local system has been detected; a judgement block 71 for judging whether a load voltage V.sub.L is larger than a reference value V.sub.ref ; a processing block 72 for selecting a load at the i-th stage when the load voltage is smaller than V.sub.ref, i.e., V.sub.L &lt;V.sub.ref ; a processing block 73 for carrying out load-breaking Pc(i), at the i-th stage where Pc(i) is set at such a value that no overcontrol takes place even when a load drop occurs due to a voltage drop; a processing block 74 for judging whether V.sub.L is larger than V.sub.ref ; a processing block 75 for selecting a load at the (i +1)th stage; a processing block 76 for calculating a total load-drop amount Pdrop by applying a method of least squares from ##EQU1## a processing block 77 for calculating a load-breaking amount Pc.multidot.ter at the final stage from Pc.multidot.ter =Ps-Pdrop; a processing block 78 for executing the load-breaking depending on the load-breaking amount Pc.multidot.ter; and a control completion block 79. It is to be noted that the Vref in the judgement blocks 71 and 74 is such a value that no load drop occurs when the load voltage V.sub.L is recovered to a value larger than the V.sub.ref, and is selected to be a value of 0.8 to 0.9 p.u. almost close to a stationary value.
Since the conventional system stabilizer is constructed as mentioned above, a voltage of a load node is needed as on-line data, and consequently the stabilizer can easily be applied to one station-one node system in the above-mentioned example. However, its application into a system having a number of load transformer substations is difficult, because each load-node voltage varies diversely after an accident has been cleared. Also, since the assumption equations of a weighting method of least squares are quite complicated as shown in Equations (1) and (2), a microprocessor of high performance is needed to achieve the method without affecting a quick response, resulting in problems such as an increase in a manufacturing cost.